1. Field of the Invention
The present invention relates to a SPECT (Single-Photon Emission Computed Tomography) apparatus, which detects gamma rays emitted from a radioisotope (hereinafter referred to as RI) introduced into a body to be examined (hereinafter referred to as target body) by means of a focus collimator and a detector in a 360.degree. direction to provide detection data, and subjects the detection data to a reconstruction process thereby to acquire a density distribution tomographic image of the radioisotope in the target body.
2. Description of the Background
There are various SPECT apparata of the above noted type classified depending on the difference in how they acquire projection data in the 360.degree. direction. For instance, one type rotates a single scintillation camera 360.degree.. There is another type which has plural sets (generally, N-numbered sets) of focus collimators and detectors and rotates 360.degree. or 360.degree./N (N being the number of sets). There is also a system which uses a fan-shaped detector Basically, however, these SPECT apparata detect gamma rays, emitted from within a tomographic slice, in a plane containing this slice and acquire projection data from a specific direction, and repeat the above processes for 360.degree. to acquire a set of data for reconstruction of an RI distribution image.
A tomographic image obtained by SPECT (which will hereinafter be referred to as a SPECT image) unless compensated with respect to the amount of an RI distribution is unsuitable for quantitative analysis, due to non-uniform degree of absorption of gamma rays in a target body (human body).
Therefore, conventionally, absorption compensation is executed in order to overcome such a shortcoming. As an example, compensation data are acquired on the assumption that a living body is a uniform material. As no living body is actually a uniform material, the degree of the compensation done by this method is not high.